1. Field of the Invention
The invention relates to a conversion process for rubber granulates, in particular resulting from shredding used tires, making it possible to produce a carbonized substance and possibly a plasticizer, which can be used in the mixtures that serve to manufacture rubber parts.
2. Description of the Related Art
The rubber industry generates significant quantities of production and usage waste. The final shape and exploitation parameters for the products are obtained during the irreversible curing process. For that reason, the recycling of rubber objects requires costly operations, demanding considerable time and work.
Generally, recycled materials have inferior physical and mechanical properties and are not competitive relative to the raw materials of the original rubber. For that reason, rubber waste is a serious problem, both economically and ecologically.
The scale of this problem may be estimated based on the production of the rubber industry, which is about 35 million tons per year. More than 150 years ago, recycling tests were conducted on rubber waste. Today, after many years, the development of suitable technologies for its elimination remains a major problem in the rubber industry. From the perspective of environmental protection, the recycling of tires is extremely important because they represent 60-70% of the rubber industry's production.
Used tires can be burned (for example in cement works, pulp and paper mills, industrial boilers). They can also be shredded mechanically to obtain rubber, textile and scrap residues. Among these, rubber residues, depending on their granulometric size, are called reground rubber or granulates, reground rubber having an average diameter smaller than 2 mm, and granulates having an average diameter of 2 to 10 mm. Rubber granulates can in particular be used in athletic clothing, sound barrier materials, etc. Reground rubber can be used in road coatings, insulating concrete, etc. Rubber granulates has the advantage of having a high rubber content (according to the specifications, for example 98%). It is very advantageous to use rubber granulate, inasmuch as its sales price is low and it also has very few outlets on the market.
The pyrolysis of tires is one of the methods developed to process used tires. The products of pyrolysis are on the one hand high-temperature gases (above 600° C.), which are generally burned to recover the available energy, and on the other hand a solid residue with a high carbon content, which is either put in a dump or used as coal.
In certain recent processes, part of the pyrolysis gas is condensed to obtain pyrolytic oils that can serve as fuel or be mixed with diesel fuel. However, the solid residue is often contaminated by compounds (textiles, scrap, sand, mineral filler of the rubber), which reduces its value and calls the profitability of the process into question.
The pyrolysis of the rubber is done at high temperatures, in general much higher than 500° C., as in U.S. Pat. No. 4,588,477 or BE 820012. Pyrolysis at a reduced pressure makes it possible to reduce the pyrolysis temperature, as explained for example in US 2003/0079664 or U.S. Pat. No. 5,229,099. However, these rubber pyrolysis processes do not make it possible to obtain a solid residue that is directly usable. In fact, the solid residue obtained has the features of a non-active coal, which has a nonexistent reinforcing power when it is used as additive in rubber mixtures.
Pyrolysis methods exist carried out from rubber granulates as raw material. Thus, international application WO02/38658 describes a process for converting tire granulates through pyrolysis within a temperature range of 400 to 950° C., preferably from 500 to 750° C. However, although the process described uses rubber granulates, WO'658 does not specify that the pyrolysis is done in the presence of water. Thus, the solid residue obtained in WO092/38658 is not described as being directly exploitable in the rubber industry.